A fluorometer is an analytical device that essentially comprises a light source, a means of selecting the desired excitation wavelength range, a sample cell, a means of selecting the desired emission wavelength range, and a detector. A spectrofluorometer is a specific type of fluorometer where the means for selecting the excitation and/or emission wavelength range is performed by a grating. A grating acts to disperse a continuum of light into its components. Spectrofluorometers may be further subdivided into scanning spectrofluorometers, those that use a mechanical means to scan the wavelength spectrum based on the position of the grating relative to the excitation source and/or emission (this describes a standard laboratory model fluorometer), or fixed spectrofluorometers where the grating is fixed with respect to the emission. The emission (fluorescence) is then directed to an array of detectors. The array of detectors could be charge coupled devices, usually abbreviated “CCD” or the array of detectors could be photodiodes. The detectors are then calibrated in the appropriate wavelength units. A commercial device such as this is available from Ocean Optics (available from Drysdale and Associates, Inc., P.O. Box 44055, Cincinnati, Ohio 45244 (513) 831-9625). This type of fixed spectrofluorometer still requires the appropriate excitation wavelength selection device, which could be a grating or filter.
The fluorometers that are most suitable for use under field conditions are not grating spectrofluorometers, rather, they are filter-based fluorometers. A filter-based fluorometer uses a filter to exclude all but the selected wavelength range. In general, currently available and known filter-based fluorometers have one channel with this channel containing an optically appropriate cell.
A light source and an optional excitation filter, are positioned on one side of the optically appropriate cell, and an emission detector and an emission filter are positioned on another side of the optically appropriate cell. A reference detector may optionally be present. Because fluorescence is isotropic, in general, fluorometers are configured to detect any fluorescent light emitted from the fluorophore at a 90° angle from the light source in order to minimize collection of any spurious excitation light.
The excitation filter permits light of the chosen excitation wavelength range to pass through the filter and into the cell. When conducting off-line batch testing, a sample of, for example, water from a natural or an industrial water system is placed and held in the optically appropriate cell. When conducting on-line testing, the sample of water can flow through the optically appropriate cell. The light is absorbed by a fluorophore present in the water sample, which, in turn, emits a fluorescent light (hereinafter known as a fluorescent signal) having the same or a longer wavelength than the excitation light. The emission filter, which is positioned between the emission detector and the optically appropriate cell, is chosen so as to permit only the light emitted by the fluorophore (the fluorescent signal of the fluorophore) to pass through the filter to the emission detector.
The use of fluorophores in industrial water systems or in hydrology in general is known. The use of inert fluorescent tracers for determining the hydraulic losses in an industrial water system is known. Furthermore, using fluorescent tracers for controlling additive or product dosage to a recirculating or once-through cooling water system is also known (see U.S. Pat. No. 4,783,314). In this method, a fluorescent tracer is combined with one or more additives in a known proportion of tracer to additive(s) and then the mixture is added to the water of a cooling system. A fluorometer is then used to detect the presence and concentration of the fluorescent tracer in the cooling water and therefore the presence and concentration of the amount of additive.
There will always be a continuing need for new and improved fluorometers to be available for use in the challenging area of monitoring and controlling industrial water processes.